With the fast developments of semiconductor process technology, the dimensions of integrated circuits (ICs) are rapidly scaled down into sub-micron level. Oxide isolation regions are usually incorporated with active areas of IC devices during the period of semiconductor processes. In general, a local oxidation (LOCOS) process is employed to form these oxide isolation regions, but the LOCOS process may induce a bird's beak structure such that the active areas of devices are unacceptably encroached. Therefore, a shallow trench isolation (STI) process is widely used to form isolation regions between active areas. The conventional process of manufacture for shallow trench isolation is shown in FIG. 1 and FIG. 2.
Referring to FIG. 1, a pad oxide layer 12 and a silicon nitride layer 14 are sequentially deposited on substrate 10, and then a shallow trench 16 is formed thereon by an etch process. A thermal oxidation process is followed to form a lining oxide layer 18 on sidewalls of the shallow trench 16. Next, a high-density plasma chemical vapor deposition (HDPCVD) is used to fill silicon oxide 19 into the shallow trench 16. The excess silicon oxide 19 over the silicon nitride layer 14 is totally removed by chemical-mechanical polishing (CMP).
Referring to FIG. 2, the silicon nitride layer 14 is stripped by hot phosphoric acid (H.sub.3 PO.sub.4) and the pad oxide layer 12 is etched away by hydrofluoric acid (HF). At the time, a silicon oxide plug 22 is remained in the shallow trench 16. When the pad oxide layer 12 is being removed, the silicon oxide plug 22 and the lining oxide layer 18 are also simultaneously etched. Generally speaking, the etch rate of the pad oxide layer 12 formed by thermal oxidation is smaller than that of the silicon oxide plug 22 formed by HDPCVD when HF is used as an etchant. Consequently, recesses 20 located on the edge of silicon oxide plug 22 will result in current leakage and a sub-threshold voltage, which evokes many problems in device operations.
Further, while the silicon oxide 19 is deposited using HDPCVD process, the crystalline structure of shallow trench 16 surface may be damaged by a lot of ions induced by high-density plasma at the start-up time of the process. Furthermore, the isolation effect of the shallow trench 16 will severely downgrade after the silicon oxide plug 22 is entirely formed.